Scanning Intrinsic Superconductivity and the Spin Hall Effect in Niobium Borides

Although niobium borides have been extensively studied on their mechanical properties, a systematic investigation on their potential metastable phases, superconducting excitations, and spin Hall effects (SHE) is still lacking. Herein, we carry out a global structure search in the Nb-B system based on the evolutionary algorithm and density functional theory. Thermodynamical, dynamical, mechanical, and quasiharmonic approximation investigations unveil four new phases, Nb3B5, Nb2B5, Nb3B7, and Nb5B4, which are promising candidates for experimental preparations. Moreover, the superconducting transitions of all Nb-B compounds are performed from electron-phonon calculations. In addition, all Nb-B compounds are predicted to be topologically nontrivial. More intriguingly, Nb2B5 is unveiled as a promising noncentrosymmetric superconductor to explore topological superconducting excitation. Furthermore, nonzero spin Hall conductivity (SHC) tensor elements of all Nb-B compounds are predicted, and NbB is predicted to own a maximal SHC value of 320 (PLANCK CONSTANT OVER TWO PI/e) (S/cm) among all Nb-B compounds. Our theoretical results fill in the gap on superconducting and SHC parameters of Nb-B binaries, demonstrating that Nb-B binaries are a potential choice to explore superconducting excitations and topological states and to investigate the SHE.